Selectivity and fidelity control



July 16, 1940. w. LYONS SELECTIVITY AND FIDELITY CONTROL Filed Jan. 18. 1937 INVENTOR WALTER LYO S ATTORNEY.

mohouhuo x9530: mamas 52585 52368 992 w n o v Patented July 16, 1940 UNITED STATES SELECTIVITY AND FIDELITY CONTROL Walter Lyons, Flushing, N. Y., assignor to Hazeltinc Corporation, a corporation of Delaware Application January 18, 1937, Serial No. 121,122

'7 Claims.

This invention relates to modulated-carrier signal receivers, and particularly to methods of, and circuit arrangements for, controlling the selectivity and fidelity of reproduction of such receivers to discriminate against undesired signals.

According to conventional radio broadcasting practice, a signal, or program, is ordinarily transmitted on a carrier frequency having two sidebands of modulation which usually extend 6 or more kilocycles on either sidethereof. Different carrier frequencies are utilized by the various broadcasting stations and these frequencies, according to present practice, are spaced throughout the broadcasting frequency ranges, usually at 10 kilocycle intervals, so that in many instances the sideband frequencies of one carrier may overlap, or closely encroach upon, those of adjacent carriers received at the same location; It is, therefore, frequently difficult to tune a receiver to a desired signal without interference from undesired signals on carrier frequencies near the desired signal, particularly when the strength of the undesired signals is comparable to, or exceeds that of, the desired signal. Socalled background noises, which are ordinarily present at the outer frequencies of the sidebands. may also interfere with satisfactory reception.

Inorder that a desired signal may be received and satisfactorily reproduced substantially free from undesired signal interference and background noises when such disturbances are present, it is necessary that some means be embodied in the receiver which is effective so to modify the selectivity and fidelity characteristics thereof as greatly to reduce the undesired signals and noise. Various arrangements have been provided for accomplishing this purpose, including adjustable band-pass selectors embodied in the carrier-frequency portion of the receiver and effective to pass a desired band of frequencies adjustable to a sufficiently narrow width to reduce the undesired signals and noise at such portion ofthe receiver. Also, tone control arrangements have been embodied in the modulation-frequency portion of the receiver, effective to attenuate the higher modulation frequencies passed by the carrier-frequency portion of the receiver in order to obtain a similar result.

Since the outer frequencies of the modulatedcarrier sidebands, in radio broadcasting, correspond to the higher notes of a program, when these frequencies are suppressed or attenuated, the fidelity of reproduction of the desired signal is correspondingly impaired, It is, therefore,

desirable that means be provided whereby the fidelity characteristics of the carrier-frequency and modulation-frequency portions of the receiver may be adjusted with facility in accordance with the conditions of reception, so that, when the relative strengths of the desired and undesired signals permit, the width of the band of frequencies passed bythe carrier-frequency portion of the receiver is such as to include the useful modulation frequencies of the desired signal and the characteristic of the modulationfrequency portion of the receiver is such that none of the useful modulation frequencies will be substantially attenuated; while, when the desired signal is weak or when undesired signals are present and of sufficient amplitude appreciably to impair reception,, the width of the band of frequencies passed by the carrier-frequency portion may be contracted and the higher modulation frequencies passed by the modulation-frequency portion may be attenuated to reduce the undesired signals and noise.

Thus, in order to obtain optimum control of the over -all fidelity characteristic of a receiver, such characteristic of both the carrier-frequency and modulation-frequency portions thereof should be adjustable. Ordinarily it is not practical or economical to design band-pass selectors WhlCh will provide the required minimum fidelity characteristic, while tone control means are designed to adjust the fidelity over an extended range. The characteristics of the tone control means, on the other hand, are ordinarily such as to effect a gradually increasing attenuation over the entire band of modulation frequencies so that, for maximum fidelity adjustments, a part of the desired higher frequencies is considerably attenuated although the cutoff is not sufiiciently sharp to attenuate high-frequency noise and modulation frequency of adjacent undesired signals to such an extent that they are not still troublesome. Moreover, in order that the required adjustments may be made with ease and facility, it is essential that the control apparatus be characterized by simplicity of construction and operation.

It is an object of the present invention, therefore, to provide an improved method of, and means for, controlling the selectivity and fidelity of reproduction of a modulated-carrier signal recelver.

It is a further object of the present invention to provide an improved method and means of the character described, whereby the fidelity characteristics of the carrier-frequency portion and modulation-frequency portion of a receiver may be successively adjusted by anoperator with ease and facility and in such manner as to provide a highly satisfactory over-all fidelity control.

In accordance with the present invention,

there is provided a modulated-carrier signal re-- ceiver which includes a carrier-frequency portion and a modulation-frequency portion, each portion having individually associated therewith means for modifying the fidelity characteristic thereof. A single control means is provided for so adjusting the two modifying means as successively to modify the characteristics of thetwo portions of the receiver.

More particularly, in the present preferred embodiment of the invention the carrier-frequency portion includes an adjustable band-pass selector and ,the modulation-frequency portion includes tone control means. There are pro vided means for adjusting the selector to contract the width of the band of frequencies passed thereby and means for, adjusting the tone con-- trol means to effect increasing attenuation of the higher frequencies passed by the carrierfrequency portion of the receiver. A single fidelity control element is utilized and means are provided which connect this element with the two adjusting means as to effect successive contraction of the band passed by the selector and increased attenuation of the higher modulation frequencies by the tone control means by movement of the control element in one direction, and vice versa.

For a better understanding of the invention, together with other and further objects thereof, reference is bad to the following description taken in connection" with the accompanying drawing, and its scope will be pointed out in the appended claims. In the accompanying drawing, Fig. 1 is a circuit diagram, partially schematic, of a complete superheterodyne receiver embodying the present invention and Fig. 2 is a diagrammatic view, partially in perspective, of an adjustable fidelity control apparatus suitable for use in connection with the invention.

Referring in detail to the drawing, the invention ishere illustrated as embodied in a superheterodyne receiver,-those parts of the receiver which are well known and do not comprise a part of the invention being indicated schematically. The receiver comprises a radio-frequency amplifier I0 having its input circuit connected with an antenna l I and a ground l2 and its output circuit connected with an oscillator-modulator l3. The output; circuit of the oscillatormodulator is connected to the input circuit of an intermediate-frequency amplifier l4, which includes an adjustable band-pass selector I5.

The selector [5 may be any suitable design and, in the embodiment illustrated, comprises a main transformer I 6 having fixed-coupled primary and secondary windings l1 and I8 and an adjustable auxiliary transformer l9 having primary and secondary windings 2D and 2| connected in series with the windings l1 and I8, respectively, be-

tween their low alternating potential ends and ground. The winding I1 is included in the output circuit of the oscillator-modulator and its connection includes a source of operating voltage for the oscillator-modulator, as indicated at +3. Adjustably fixed condensers 22 are connected across the primary and secondary circuits of the selector, as illustrated. The output circuit of the selector is connected to the input circuit of selectively to amplify the signal.

a suitable number of stages of intermediatefrequency amplification Ma. I

The output circuit of the intermediate-frequency amplifier stages Ila is connected to a detector 23, the latter having, its output circuit in turn connected to the input circuit of an audio-frequency amplifier 24. The audio-frequency amplifier is coupled to a loud-speaker 26 by way of an impedance network 3| and an audio-frequency transformer 21. The primary winding of transformer 27 is included in the output circuit of the audio-frequency amplifier 24, which circuit includes a source of operating voltage, indicated at +B. The impedance network or tone control circuit'3l, which forms a part of the present invention, includes a condenser 32 in series with an adjustable resistor 33, connected across the output circuit of the amplifier 24. An automatic amplification control conductor 25 is connected to the A. V. C. rectifier or detector 23 and the unidirectional voltage developed thereby is applied by the conductor 25 to the control electrodes of one or more of the amplifier tubes in the preceding stages of the receiver, in accordance with conventional practice. Neglecting for the moment the fidelity control provided by the selector l5 and tone control circuit 3!, the system described above comprises a conventional superheterodyne receiver, the operation of which is well understood in the art, so that a detailed explanation thereof is deemed unnecessary. In brief, however, signals intercepted by the antenna II are selected and amplified in the radio-frequency amplifier Ill and supplied to the oscillator-modulator 13, wherein the signals are converted to an intermediate frequency in the usual manner. The output of the oscillator-modulator is delivered to the intermediate-frequency amplifier 14, wherein the selector system l5 serves to select the desired band of frequencies and the stages Ma serve further The amplified intermediate-frequency signalis thereupon supplied to the detector 23 wherein the modulation or audio frequencies .are derived and passed on to ,the audio-frequency amplifier 24 for amplifica tion in the usual manner and, by way of the impedance network 25, to the loud-speaker 26 for reproduction. The automatic amplification control means functions in the well-known manner to maintain the signal input to the detector 23 within a narrow range for a wide range of received signals. a

While, in accordance with the present invention, any desired type of adjustable band-pass selector and any desired type of tone control circuit maybe employed, the particular arrangements of the preferred embodiment of the inven tion just described have been found highly satisfactory. The auxiliary windings 20 and'2l of the selector preferably have a self-reactance which is a small fraction of that of the main coupling windings I1 and I8 so that the coupling therebetween is adjustable over a relatively wide range to vary the coupling between the circuits of the selector over a substantially lesser range, thereby providing, in effect, a Vernier control of the selectivity and fidelity of this portion of the receiver. The tone control means provided by the condenser 32 and adjustable resistor 33 serves to attenuate the higher modulation frequencies passed by, the carrier-frequency portion of the system by by-passing such frequencies to ground to a greater or lesser extent. Specifically, de-

creases in the value of the resistor 33 increase the lay-passing or attenuating of the higher audio frequencies.

In order that the fidelity characteristics of the carrier-frequency and modulation-frequency portions of thesystem may be adjusted with ease and facility and in the most desirable manner,- there is provided a unicontrol device indicated in Fig. 1 by the broken line U. This device or single control is so constructed and arranged that movement thereof in one direction effects successive adjustment of the auxiliary coupling transformer l9 to contract the pass-band of the selector and adjustment of the adjustable resistor 33 to effect an increased attenuation by the tone control means of the higher frequencies passed by the carrier-frequency portion of the receiver. In this manner the over-all fidelity of the receiver may be adjusted from maximum to minimum within predetermined limits by the successive operation of the two adjusting means by a single control. In readjusting the unicontrol device from a position providing minimum over-all fideity to one providing maximum over-all fidelity, the two individual adjusting means are, of course, successively actuated in opposite order, so that first the fidelity characteristic of the audio-frequency portion is improved and then the fidelity characteristic of the carrier-frequency portion.

While in accordance with the present invention any suitable means, either electrical or mechanical, may be provided as the unicontrol device, one

. example of a mechanical unicontrol arrangement is illustrated in Fig. 2. According to this arrangement, the movable winding 20 of the auxiliary transformer i9 is mounted within the stationary winding M on a section 34a of a rotatable shaft 34. The windings 20 and 2! are angularly related to the shaft so that, by a rotation of the latter, the winding 20 may be adjusted between a position with its axis coincident to the axis of the winding 2i, providing a maximum amount of coupling, and a position with its axis perpendicular to the axis of the winding 2i effecting a minimum amount of coupling. The shaft 34 also includes a section 34b, to one end of which an operating knob 35 is secured. The resistor 33 is of the well-known ring construction of rheostat provided with an adjustable tap or arm 36 mounted on the shaft section 34b for, angular movement therewith, one section 33a of the resistor 33 being such as to afford no appreciable resistance. Rotation of the shaft 34 from the position shown in Fig. 2 and in the direction indicated by the arrow thus effects substantially no change of the value of the resistance in the tone control circuit during the first 180 degrees of movement but thereafter short-circuits an increasingly greater portion of the resistor 33, ,increasingly to attenuate the higher modulation frequencies and, hence, gradually to decrease the fidelity characteristic of the modulation-frequency portion of the receiver.

The adjacent ends of the two shaft sections 3412 and 34?) are coupled by means of a lost motion device 31. This device may comprise a fixed annular member 38 having rotatably disposed therein a drum 39 to which the shaft section 34a is rigidly secured for rotation therewith. The end of theshaft section 34b is frictionally engaged in an axial opening in the drum 39, so that rotation of the section 34b normally eifects rotation of the drum and, hence, the shaft section 34a and winding 20. An arm 40, however, is so mounted on of the drum '39, shaft section 34a and winding 29 -only from the position illustrated,'corresponding to maximum coupling of the auxiliary windings 20 and II, through an angle of approximately 180 degrees, corresponding to minimum coupling. Further rotation of the shaft section 341), however, is permitted by its frictional engagement with the drum 38. The stop 42 corresponds to the position in which the contact 38 is at the junction of the section 33a and the resistor 33 proper. Hence, upon rotation of the knob 33 beyond 180 degrees, 1. e., beyond the point where the selector is adjusted for maximum selectivity, the tone control comes into operation. Briefly, rotation of the knob 35 from the position illustrated and in the direction indicated by the arrow first effects an adjustment of the band-pass selector gradually to decrease the fidelity characteristic of the carrier-frequency portion of the receiver, and then adjusts the tone control means gradually to decrease the fidelity characteristic of the modulation-frequency portion of the receiver.

In order that, upon adjustment of the control system in the opposite sense by rotation of the knob 35 in a direction opposite to that indicated by the arrow fromthe position corresponding to minimum over-all fidelity, the tone control means will first be adjusted and then the band-pass selector, the lost motion device 31 is provided with means for preventing movement of the drum 39 in the member 38 for the initial I80 degree movement of the shaft section 34b. To this end there is provided a sliding pin 43 disposed in a radial passage formed in the drum 39 and biased inwardly by a spring 44. An indent 45 is formed in the shaft section 34b and normally receives the inner end of the pin. A socket 43 is formed in the inner wall of the annular member 38 at such a position that, when the arm 40 contacts the stop 42, the pin 43 is opposite this socket. Further rotation of the drum 39 being thus prevented, any further rotation of the shaft section 34b acts to force the pin 43 outwardly by virtue of the carnming action between the indent and the inner end of the pin, so that the outer end of the pin enters the socket 46 and locks the drum 39 to the fixed member 38. Thus, when the shaft section 34?: is rotated to any position beyond that at which the pin 43 enters the socket 46, a reverse rotation thereof, regardless of the frictional engagement between this shaft section 34b and the drum, cannot effect rotation of the drum. Hence, in adjusting the over-all fidelity characteristic from minimum to maximum the tone control means will always be adjusted first and the band-pass selector will not be adjusted until the shaft section '34!) has been rotated to a position where the indent 43 is so disposed as to permit the pin to be forced by the spring radially inward and out of the socket 46, thereby unlocking the drum 39 from the ring 38. The selectorv i will then be adjusted to increase the fidelity characteristic of the carrier-frequency portion of the receiver.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.

What is claimed is: I

1. A modulated-carrier signal receiver com prising a carrier-frequency portion, a modulation-frequency portion, means individually associated with each of said portions for modifying the fidelity characteristic thereof, and a single control means for adjusting said modifying means successively to modify the fidelity characteristics of said two portions of the receiver.

2. A modulated-carrier signal receiver comprising a carrier-frequency portion, a modulation-frequency portion, means individually associated with each of said portions for adjusting the efiect thereof on the over-all fidelity of the system, and a single control means connected to operate said adjusting means to'adjust said overall fidelity from maximum to minimum within predetermined limits by successive operation of said carrier-frequency adjusting means and said modulation-frequency adjusting means.

3. A modulated-carrier signal receiver comprising a carrier-frequency portion including a band-pass selector, a modulation-frequency portion including a tone control means, means for adjusting said selector to contract the width of the band of frequencies passed thereby, means for adjusting said tone control means to effect increasing attenuation of the higher frequencies passed by said modulation-frequency portion, and

a single control means for successively operating said two adjusting means in the order named.

4. A modulated-carrier signal receiver comprising a carrier-frequency portion including a band-pass selector, a modulation-frequency por-- tion including tone control means, means for adjusting said selector to contract the width of the band of frequencies passed thereby, means for adjusting said tone control means to effect increasing attenuation of the higher frequencies passed by said modulation-frequency portion, a single control element, and means connecting said element with said two adjusting means to effect successive contraction of said band by said selector and increased attenuation of said higher frequencies by said tone control means by movement of said element in a single direction.

5. A modulated-carrier signal receiver comprising a carrier-frequency portion, 'a modulation-frequency portion, means individually associated with each of said portions for modifying the fidelity characteristic thereof, and a single control means so connected with said modifying means that, by movement of said control means in one direction, fidelity adjustment is efiected by only one of said modifying means over a predetermined range of said movement and fidelity adjustment is effected only by the other of said modifying means over the remainder of said movement.

6. The method of controlling the over-all fidelity of a modulated-carrier signal receiver having a carrier-frequency portion and a modulationfrequency portion which comprises first adjusting the fidelity characteristic of the carrier-frequency portion from maximum to minimum within. 

